A panel of experts recommends that the U.S. space agency NASA not rule out sending astronauts to repair the Hubble Space Telescope. Their call contradicts NASA's strong stand against launching a shuttle to upgrade the orbiting observatory.

A panel of scientists and engineers appointed by the U.S. National Academy of Sciences urges NASA to refurbish the observatory one final time, predicting that compelling scientific returns will result.

NASA had scheduled a space shuttle mission for 2006 to replace Hubble's weakening batteries and stabilizing gyroscopes and add two new instruments to expand its discovery capability 10 to 30 times.

But space agency chief Sean O'Keefe canceled the flight after the loss of the orbiter Columbia and its seven crewmembers last year, saying it would entail risks NASA is not willing to take. He said that when shuttles return to flight after safety modifications sometime next year, they would fly only to complete international space station construction.

A strong public outcry forced him to reconsider his decision, but he has committed only to the possibility of a robotic repair mission to the Hubble.

However, the National Academy of Sciences panel says NASA should not forego the astronaut option as it pursues the robot plan.

The panel's chairman is Louis Lanzerotti, a consultant with Lucent Technologies.

"The current uncertain status of the shuttle return to flight as well as uncertainties in the early stages in the development of a robotic mission lead us to believe that the key technical decision points are at least a year in the future and that keeping both options open would be a prudent direction for the agency to take," Mr. Lanzerotti said.

The chairman of a congressional committee that oversees NASA said he endorses the panel's recommendation.

NASA has conducted a strong campaign not to dispatch a shuttle to the Hubble Telescope. It cites the experts who investigated the Columbia accident as saying shuttles face more risk if they fly anywhere other than to the space station. This is partly because astronauts would not have the safe haven the station can provide if a shuttle were to have potentially dangerous technical problems in orbit.

NASA's Sean O'Keefe told the National Academy of Sciences panel recently that if a shuttle were in trouble at the telescope, launching another shuttle to rescue the crew would be very risky.

"The consequences of getting it wrong on a shuttle servicing mission to which you are at the worst scenario and have to effect this mid-orbit transfer that we've never tried before, I think I'd rather be in a position of trying a robotic autonomous rendezvous and docking capacity that we've never tried before rather than a human transfer between two orbiters that we've never tried before on the unlikely event that that were to happen," he noted.

But the National Academy of Sciences experts remind Mr. O'Keefe that he also testified that eliminating risk in space is impossible. In addition, they point out that the Columbia accident investigators found it acceptable to take fewer risk reducing steps to shuttle destinations other than the space station simply because there are fewer measures that can be taken.